How can I achieve more realism in this given scene?
Work on the details and imperfections(cracks,  scratches, etc.). 
Add more assets as you can.  I believe there is no car wash that is sitting alone in a parkway. From simple things like debris and trash cans to more complex scenery like cars and humans will make your scene complete. 

Try to reduce low-quality textures for the building and try to model it yourself using procedural materials. 90% of realism is focused on lighting, materials, and scaling.

And I don't think StackExchange is a good place for critics and specific requests, try Blender Artists instead.